1. Field of the Invention
The present invention relates to a process for producing dichlorodiketopyrrolopyrrole pigment of the α modification, dichlorodiketopyrrolopyrrole pigment of the α modification produced by the process, a coloring composition using the same, and a color filter containing the coloring composition. More particularly, it relates to a process for producing dichlorodiketopyrrolopyrrole pigment of the α modification which is size-controlled to fine particles with a uniform particle size and has extremely good dispersibility, dichlorodiketopyrrolopyrrole pigment of the α modification which is produced by this process, which has the aforementioned properties, which can impart excellent viscosity properties to a coloring composition and which, when used for a color filter to be used in a color liquid crystal device and in a solid photographing element, can form a uniform colored film having a higher contrast ratio, thus providing a color filter having excellent display quality, i.e., high lightness, high distinctness and high transmittance, a coloring composition containing the pigment, and a color filter containing the coloring composition.
2. Description of Related Art
Dianthraquinone pigments, perylene pigments, diketopyrrolopyrrole pigments, and the like have conventionally been used for producing a red filter segment of a color filter. In particular, diketopyrrolopyrrole pigments are predominantly used as red pigments for a color filter owing to their high lightness, excellent light fastness and excellent heat resistance. As use of liquid crystal display devices has been enlarged from monitors to large-sized color television sets in recent years, color filters have become required to have a high contrast ratio. As a result, pigments are required to be finer and more size-controlled particles than those of pigments which are commonly used in printing inks, gravure inks and coloring agents. In order to meet the requirement, it is necessary to more reduce and more control the primary particle size of the pigments.
With some organic pigments such as azo pigments, it is possible to obtain fine and size-controlled particles by properly selecting reaction conditions upon synthesis thereof. However, there are those pigments such as copper phthalocyanine green pigment which are generated upon synthesis as extremely fine and agglomerated particles and are then subjected to an after-treatment to cause growth of particles and to control size of the particles, and those pigments such as copper phthalocyanine blue pigment which are generated upon synthesis as coarse and irregular particles and are then subjected to an after-treatment to reduce and control the particle size, thereby forming a pigment product. The diketopyrrolopyrrole pigments are generally synthesized by the reaction of a succinic acid diester with an aromatic nitrile in an organic solvent (see JP-A-58-210084). The crude diketopyrrolopyrrole pigment is heat-treated in water or in an organic solvent, and then subjected to pulverization such as wet milling to convert into a form adequate for use (see JP-A-5-222314).
Of the diketopyrrolopyrrole pigments, C.I. Pigment Red 254, a dichlorodiketopyrrolopyrrole pigment, is the pigment predominantly used for a color filter. As C.I. Pigment Red 254, α and β modification are known (see JP-A-8-48908). In JP-A-8-48908, the β modification is defined as a novel crystal modification of the dichlorodiketopyrrolopyrrole pigment while a crystal modification known before filing of the patent application is described as the α modification. It is also described therein that the β modification is obtained by synthesis of dichlorodiketopyrrolopyrrole. X-Ray diffraction charts of the α modification and the β modification are also known (see High Performance Pigments, WILEY-VCH, pp. 174-175) and, in general, it is known that the α modification is a stable system, whereas the β modification is an unstable system. The crystal modification of dichlorodiketopyrrolopyrrole pigment conventionally used for a color filter is the α modification.
However, since the dichlorodiketopyrrolopyrrole pigment of the α modification is produced through a step of heat-treating in water or in an organic solvent, the pigment particles have a large particle size. Therefore, dichlorodiketopyrrolopyrrole pigment of the α modification comprising fine and size-controlled particles have been unable to be obtained even by wet milling the dichlorodiketopyrrolopyrrole pigment of the α modification.
Further, in the case of wet milling the dichlorodiketopyrrolopyrrole pigment of the β modification, it is necessary to convert the crystal modification from the β modification to the α modification by wet milling. Therefore, in order to obtain fine dichlorodiketopyrrolopyrrole pigment of the α modification from dichlorodiketopyrrolopyrrole pigment of the β modification, both a period of crystal transfer step and a period of pulverizing step are required. Further, since crystal transfer is involved, the pigment particle size immediately after the conversion is larger than before performing the wet milling, thus efficiency being extremely bad.